1. Field of the Invention
The present invention relates to a process for the polymerization of ethylene.
More specifically, the present invention is concerned with a process for the production of ethylene polymers having a melt flow rate (hereinafter abbreviated as MFR) controlled to a low value, which is primarily characterized in the catalyst to be used.
2. Statement of the Prior Art
The greatest problem with the process for producing polyethylene is the removal of heat, because the polymerization of ethylene is an exothermic reaction. Polymerization of ethylene at a higher temperature results in an increase in the efficiency of removal of heat, since there may be a larger difference in temperature between the temperature of the polymerization mass and the temperature of the coolant due to the elevated reaction temperature.
Polyethylene is generally used in the form of pellets. However, suspension polymerization and gas-phase polymerization give powdery polyethylenes, which have to be pelletized through melting. If such polymerization is carried out at a higher temperature, for instance, a temperature higher than the melting point of a polymer product, it would be very advantageous in view of energy, since as-molten polyethylene can be introduced directly into an extruder for pelletizing.
On the other hand, a problem with the application of higher temperatures to polymerization is that it may not be possible to obtain polymer products having a melt flow rate or index (MFR) reduced to a sufficiently low region, since the chain transfer rate is increased more remarkably than the growth rate of ethylene at such higher temperatures. This means that further difficulty is encountered in reducing the MFR of copolymers of ethylene with .alpha.-olefins due to the fact that the chain transfer rate of .alpha.-olefins is increased over that of ethylene.
The point as discussed above places an upper limit on the polymerization reaction temperature in view of the desired MFR of polymers, and further, leads to some limitation being imposed upon the degree of conversion of polymers. Thus, of vital importance is the development of catalysts capable of attaining a wide range of MFR's or lower MFR's over a sufficiently high temperature range.
As means for providing a solution to such problems, Japanese Patent Laid-Open Publication Nos. 59(1984)-14 5206, 61(1986)-204204, 61(1986)-207405, 61(1986)-276803 and 62(1987)-84107 have proposed catalysts comprising combinations of a solid catalyst component containing at least titanium, magnesium and a halogen as the transition metal component; an organoaluminium compound serving as an organic metal component; and a third component to be added at the time of polymerization.
In addition to such catalysts, those comprising a metallocene compound of a transition metal and an aluminoxane have recently been proposed as new catalysts for the polymerization of olefins (Japanese Patent Laid-Open Publication Nos. 58(1983)-19309, 60(1985)-35006, 60(1985)-35007, 60(1985)-35008, 60(1985)-130604, 60(1985)-217209, 60(1985)-245604, 60(1985)-245605, 61(1986)-31404, 61(1986)-130314 and 61(1986)-276805).
These catalysts are homogeneous polymerization catalysts, while conventional catalysts such as those based on titanium trichloride and titanium-on-magnesium are non-homogeneous polymerization catalysts. For that reason, when the copolymerization of ethylene with .alpha.-olefins is carried out with such homogeneous polymerization catalysts, there is an advantage that the obtained polymers differ in copolymerizability from ethylene copolymers prepared with the conventional non-homogeneous polymerization catalysts.
It has now been found, however, that when polymerization is carried out at a further elevated temperature with the above homogeneous polymerization catalysts, there is a problem that the MFR of polymer products is increased to an unacceptable degree.